1. Field of the Invention
The present invention relates to an exhaust emission control system for an engine, and a method of controlling the exhaust emission control system.
2. Description of the Background Art
As an exhaust emission control system for purifying nitrogen oxides (NOx) contained in exhaust gas of an engine, there has been known one type using ammonia as a reducing agent for a catalyst for selective catalytic reduction (SCR) of NOx (hereinafter referred to as “NOx SCR catalyst”). In this type of exhaust emission control system, an aqueous urea solution is added into exhaust gas in an exhaust passage and hydrolyzed by heat of the exhaust gas, and ammonia produced by the hydrolysis is supplied to and absorbed by a NOx SCR catalyst disposed in a downstream region of the exhaust passage. Through this process, a denitration reaction between the ammonia and NOx contained in the exhaust gas is promoted to purify the NOx.
On the other hand, the above exhaust gas emission control system has some problems.
A first problem is so-called “ammonia slip”. In view of enhancing NOx conversion efficiency, it is desirable to allow ammonia to be absorbed by a NOx SCR catalyst in a larger amount. However, the NOx SCR catalyst has limitations in an absorption amount of ammonia. If ammonia is excessively supplied to a NOx SCR catalyst, a part of the ammonia will pass through the NOx SCR catalyst as unreacted or slipped ammonia, and the slipped ammonia will be released directly into the atmosphere, i.e., ammonia slip will occur.
A second problem is an allowable ambient temperature range for an aqueous urea solution. Generally, a concentration of urea in the aqueous urea solution is set at a value which allows a freezing temperature of the aqueous urea solution to become the lowest value. However, the lowest freezing temperature is −11° C. (=12° F.) at best. Thus, considering that an ambient temperature in cold regions often drops below the lowest freezing temperature, there remains a high risk of freezing of the aqueous urea solution in such low-temperature environments. If an aqueous urea solution remaining in an aqueous-urea-solution supply passage is frozen, resulting volumetric expansion thereof is likely to cause burst of the aqueous-urea-solution supply passage. Reversely, in high-temperature environments, water contained in an aqueous urea solution remaining in the aqueous-urea-solution supply passage will be vaporized to cause an increase in concentration of urea in the resulting aqueous urea solution. Thus, the freezing temperature of the resulting aqueous urea solution becomes higher to cause a higher risk of freezing in the low-temperature environments. As above, an allowable ambient temperature range for the aqueous urea solution is narrow, and there is a need for measures against this problem
As measures against freezing of an aqueous urea solution remaining in the aqueous-urea-solution supply passage, it is contemplated to install a heater in the aqueous-urea-solution supply passage or use an additive capable of lowering the freezing temperature. As another measure, JP 2008-101564A (hereinafter referred to as “Patent Document 1”) proposes a technique of, after stopping an engine, driving a pump to suck back an aqueous urea solution remaining in an aqueous-urea-solution supply passage, so as to recover the aqueous urea solution to an aqueous-urea-solution tank. This technique is excellent in terms of capability to prevent damage of the aqueous-urea-solution supply passage.
Recent years, a vehicle having a function of automatically stopping an engine, for example, during stop of the vehicle at a red light, and automatically restarting the engine in response to detection of a driver's operation for starting moving the vehicle, so-called “idling stop (automatic engine stop/start) function”, as one exhaust emission control function, has become increasingly popular.
However, if an exhaust emission control system is designed to have both the idling stop function, and the aqueous-urea-solution recovery function as disclosed in the Patent Document 1, the following problems will occur.
Typically, the aqueous-urea-solution recovery operation as disclosed in the Patent Document 1 is required to take about several tens of seconds. On the other hand, as might be expected, a time-period after an engine is automatically stopped through until the engine is automatically restarted (a time-period between an automatic engine stop and an automatic engine restart) according to the idling stop function is not constant. Thus, there is a possibility that, after the pump is driven to start recovering an aqueous urea solution remaining in the aqueous-urea-solution supply passage, in response to the automatic engine stop, the engine is automatically restarted within a short time-period (e.g., several seconds). In this case, the pump driving for the recovery operation is unnecessary in terms of preventing damage of the aqueous-urea-solution supply passage, i.e., anti-freezing.
Moreover, in this case, a part of the aqueous urea solution remains in the aqueous-urea-solution supply passage without being recovered. Then, in response to the automatic engine restart, the pump is driven to start supplying an aqueous urea solution so as to pre-fill the supply passage with the aqueous urea solution. During this operation, it is necessary to supply the aqueous urea solution while allowing air in the supply passage to be released therefrom. Specifically, it is common practice to open an aqueous-urea-solution addition value in response to start of the pre-filling operation, and then close the aqueous-urea-solution addition value when an amount of the supplied aqueous urea solution reaches a predetermined amount (e.g., 80% of the entire volume of the supply passage is pre-filled with the aqueous urea solution). Thus, if the aqueous urea solution remains in the aqueous-urea-solution supply passage as described above, a leakage of the aqueous urea solution from the aqueous-urea-solution addition value occurs, which is likely to cause ammonia slip.
These are problems in an exhaust emission control system having both the idling stop function and the aqueous-urea-solution recovery function.